1. Field of the Invention
The present invention relates to a liquid crystal display (LCD) device and more particularly, to an array substrate for In-Plane Switching (IPS) mode liquid crystal display device which displays superior images by preventing light leakage in an area adjacent to a thin film transistor region.
2. Discussion of the Related Art
A typical liquid crystal display (LCD) device uses optical anisotropy and polarization properties of liquid crystal molecules. The liquid crystal molecules have a definite orientation order in alignment resulting from their thin and long shapes. The alignment direction of the liquid crystal molecules can be controlled by supplying an electric field to the liquid crystal molecules. In other words, as the alignment direction of the electric field is changed, the alignment of the liquid crystal molecules also changes. Because incident light is refracted to the orientation of the liquid crystal molecules due to the optical anisotropy of the aligned liquid crystal molecules, image data is displayed.
Active matrix liquid crystal display (AMLCD) devices, in which the thin film transistors and the pixel electrodes are arranged in the form of a matrix, are widely used because of their high resolution and superiority in displaying moving images. An array substrate for the in-plane switching (IPS) mode liquid crystal display (LCD) device will be described hereinafter with reference to the following figures.
FIG. 1 is a plan view of a pixel of an array substrate for a related art in-plane switching (IPS) mode liquid crystal display (LCD) device. In FIG. 1, a plurality of gate lines 12 and common lines 16 are horizontally formed on an array substrate 10 and are spaced apart from each other. A plurality of data lines 24 is vertically formed on the array substrate 10 and cross the gate lines 12 and the common lines 16. The data line 24 defines a pixel region “P” by crossing the gate line 12. A thin film transistor “T” is formed at a crossing point of the gate line 12 and the data line 24. The thin film transistor “T” includes a gate electrode 14, an active layer 20, a source electrode 26 and a drain electrode 28. The gate electrode 14 communicates with the gate line 12 and the source electrode 26 communicates with the data line 24. The source electrode 26 has a U-shape surrounding the drain electrode 28 and the drain electrode 28 has an I-shape. A pixel electrode 30 that communicates with the drain electrode 28 and a common electrode 17 parallel with the pixel electrode 30 are formed in the pixel region “P”. The common electrode 17 communicates with the common line 16. The pixel electrode 30 comprises an extension portion 30a, a vertical portion 30b and a horizontal portion 30c. The extension portion 30a of the pixel electrode 30 is extended from the drain electrode 28 and the vertical portion 30b of the pixel electrode 30 is vertically extended from the extension portion 30a. The horizontal portion 30c of the pixel electrode 30 is formed over the common line 16 and connects the vertical portions 30b into one portion. The common electrode 17 comprises a horizontal portion 17a and a plurality of vertical portions 17b. The plurality of the vertical portions 17b of the common electrode 17 is arranged in an alternating order with the vertical portions 30b of the pixel electrode 30. The horizontal portion 17a of the common electrode 17 connects the plurality of the vertical portions 17b into one portion. The vertical portions 17b of the common electrode 17 are spaced apart from the data line 24. A storage capacitor “C” is formed in the pixel region “P”. The storage capacitor “C” uses a portion of the common line 16 as a first storage electrode and the horizontal portion 30c of the pixel electrode 30 as a second storage electrode.
A typical driving method of a pixel will be described hereinafter with reference to FIG. 2 which is a signal graph illustrating voltage signals of a gate electrode and a common electrode during a frame. The graph shows applying states of a gate voltage (Vg) and a common voltage (Vcom) during a frame period. In FIG. 2, Vgh and Vgl show a high and a low state of the gate voltage Vg and Vdh respectively, and Vdl show a high and a low state of the data voltage Vd. As shown in the figure, a voltage of about +18 volts is applied when the gate voltage (Vg) is in the on state and a voltage of about −5 volts is applied when the gate voltage (Vg) is off state. A high-level data voltage (Vd) is inputted when the gate voltage (Vg) is in the on state and maintained until a next gate voltage (Vg) reaches the on state. This voltage applying process fulfills a drive of liquid crystal in the pixel region. If the liquid crystal display (LCD) device is normally black mode, a black color is displayed when the voltage is not applied. However, a light leakage phenomenon occurs in region “A” of FIG. 1 when the gate voltage (Vg) is in the off state.
FIG. 3 is a plan view illustrating an electric field direction between the gate electrode and the common electrode when the gate voltage (Vg) is in the off state according to the related art. FIG. 4 is a plan view illustrating an alignment direction of liquid crystal molecules between the gate electrode and the common electrode when the gate voltage (Vg) is in the off state according to the related art.
In FIG. 3, region “B” shows an electric field distribution in a region “E” between the gate electrode 14 and the common electrode 17. In FIG. 4, region “F” is a long axis direction of the liquid crystal 19 and region “D” is a rubbing direction of a substrate. As shown in FIG. 4, the liquid crystal 19 is aligned parallel with the rubbing direction when the voltage is not applied. However, an electric potential difference actual exists in the region “A” between the gate electrode 14 and the common electrode 17. Subsequently, the electric field distribution occurs, which has a certain direction, between the gate electrode 14 and the common electrode 17. As shown in FIG. 3 and FIG. 4, the electric field direction is perpendicular to the long axes “F” of the liquid crystal 19 in the region between the gate electrode 14 and the common electrode 17. The liquid crystal 19 will subsequently align according to the electric field direction when the gate voltage (Vg) is in the off state. Accordingly, light irradiated from a backlight passes through the “A” region and thus the light leakage phenomenon occurs when the gate voltage (Vg) is not applied at the normally black mode. Though a black matrix is generally formed on an upper substrate (not shown) in order to intercept the light leakage in the region “A”, it still may be difficult to display a high quality image if an aligning error of the upper and lower substrate occurs.